Light emitting devices including light emitting diodes (LEDs) are well known solid devices that generate light having a peak wavelength in a specific region of the light spectrum. LEDs are typically used as illuminators, indicators, and displays. LEDs have been developed to emit light in a relatively narrow band around a peak wavelength. Light having a first peak wavelength (“primary light”) can be converted into light having a longer peak wavelength (“secondary light”) using a process known as luminescence. The luminescent process involves absorbing the primary light by a photoluminescent phosphor material, which excites the atoms of the phosphor material, and emits the secondary light. The peak wavelength, and the band of wavelengths around the peak wavelength, of the secondary light will depend on the phosphor material.
High flux density solid state light generators are now used in many applications to replace conventional light sources. Some conventional high flux density solid state light generators use light radiation emitted by LED elements in dice form (integrated circuit chips) directly. In many applications, high flux density light generators require very specific peak wavelength emission by the primary light source. Currently, LEDs can only supply peak wavelengths in the spectral bands of 420-460 nm, 520-540 nm, 620-660 nm, and a phosphor converted false white spectrum made from combining blue LEDs with a YAG (Yttrium, Aluminum, and Garnett) phosphor. It has been difficult, however, to maintain a constant correlated color temperature radiation from LED light sources.
Phosphorescent LEDs have a luminescent phosphor coating applied on top of a light emitting surface of the LED die. Generally, however, it is difficult to apply the luminescent phosphor coating uniformly on the die such that the primary light emitted from the LED has the same constant correlated color temperature, e.g., the same warmness, as a function of angle. A varying correlated color temperature is manifested when viewing the LED straight on and then from a wide angle of 80° or 90°. When viewed head on, the light will appear to have one color temperature, but when viewed from a greater angle, 80° or 90° angle from straight on, the light will appear to have another color temperature. Thus, conventional luminescent phosphor (phosphorescent) LEDs cannot produce a constant correlated color temperature radiation.